In doing some work with data and date ranges, Leonid Mamchenkov came across something strange about the "weeks" that the date and time functions in PHP work on. Sometimes, there's not always 52 weeks in a year.
With code as simple and straight-forward as [our example], you'd probably look for the problem elsewhere. Maybe it's your statistical data which is wrong, or the graph is not generated properly. But the problem is here.
How many weeks do you think there are in a year? A common knowledge says 52. However, if you think for a moment about how the weeks are related to the year, you'll realize that the first and last weeks don't necessary start and end at the edge of the year.
So, when using the date function, you might not get quite what you were expecting. It works more correctly and can include the extra "week" if there's enough of an overlap. The "W" parameter (for "week") works off of the ISO-8601 week number for the year, not the general calendar weeks.